High modulus, toughened one-component epoxy structural adhesives with high aspect ratio fillers

ABSTRACT

One-component toughened epoxy structural adhesives contain fibrous mineral fillers that have an aspect ratio of ≥6. The presence of the fibrous filler leads to a significant increase in elastic modulus while having little if any adverse effect on other properties such as dynamic impact peel resistance.

This invention relates to one-component epoxy structural adhesives thatcontain a toughener.

Toughened one-component epoxy structural adhesives are used extensivelyin the automotive and other industries for metal-metal bonding as wellas bonding metals to other materials. Often, these structural adhesivesmust strongly resist failure during vehicle collision situations.Structural adhesives of this type are sometimes referred to as “crashdurable adhesives”, or “CDAs”. This attribute is achieved through thepresence of certain types of materials in the adhesive formulation.These materials are often referred to as “tougheners”. The toughenershave blocked functional groups that, under the conditions of the curingreaction, can become de-blocked and react with an epoxy resin.Tougheners of this type are described, for example, in U.S. Pat. Nos.5,202,390, 5,278,257, WO 2005/118734, WO 2007/003650, WO2012/091842, U.S. Published Patent Application No. 2005/0070634, U. S. Published PatentApplication No. 2005/0209401, U. S. Published Patent Application2006/0276601, EP-A-0 308 664, EP 1 498 441A, EP-A 1 728 825, EP-A 1 896517, EP-A 1 916 269, EP-A 1 916 270, EP-A 1 916 272 and EP-A-1 916 285.

These tougheners are highly effective in imparting toughness, asindicated by the ISO 11343 dynamic impact peel strength test. However,the presence of the tougheners greatly reduces the elastic modulus ofthe cured adhesive. The elastic modulus refers to the slope of thestress-strain curve of the cured adhesive when subjected to tensiletesting using methods such as DIN EN ISO 527-1. Because of the lowelastic modulus of the cured adhesive, the bonded assembly of substratesand adhesive is more flexible (i.e., less stiff) than is wanted.

It is common to incorporate a particulate filler into structuraladhesives of this type, for rheological reasons and to decrease cost perunit weight. However, this has been found to impart only a modestincrease in elastic modulus, and can result in a significant loss ofdynamic impact peel strength.

What is desired is a one-component, structural adhesive that when curedexhibits high dynamic impact peel strength and a high elastic modulus.Preferably, these benefits are obtained without significant increase inthe viscosity of the uncured adhesive.

This invention is a one-component epoxy structural adhesive comprisingin admixture:

A) at least one epoxy resin;B) a reactive toughener containing urethane and/or urea groups andcapped isocyanate groups;C) one or more epoxy curing agents;D) 1 to 40 weight percent, based on the total weight of the epoxystructural adhesive, of a mineral filler in the form of fibers having adiameter of 3 μm to 25 μm and an aspect ratio of at least 6, andE) one or more epoxy curing catalysts,wherein the curing agent(s) and epoxy curing catalyst(s) are selectedtogether such that the structural adhesive exhibits a curing temperatureof at least 60° C.

The presence of the fibrous filler having the specified dimensions hassurprisingly been found to impart a substantial increase in the elasticmodulus of the cured adhesive. Surprisingly, this benefit is achievedwithout significant loss of dynamic impact peel strength ordeterioration of other important properties.

The invention is also a method comprising forming a layer of thestructural adhesive of the invention at a bondline between twosubstrates, and curing the layer to form an adhesive bond between thetwo substrates. At least one and preferably both of the substrates maybe metals.

Suitable epoxy resins include those described at column 2 line 66 tocolumn 4 line 24 of U.S. Pat. No. 4,734,332, incorporated herein byreference. The epoxy resin should have an average of at least 1.8, morepreferably at least 1.9 epoxide groups per molecule. Preferably, atleast a portion of the epoxy resin(s) is not rubber-modified, meaningthat, prior to curing, the epoxy resin is not chemically bonded to arubber.

Suitable epoxy resins include diglycidyl ethers of polyhydric phenolcompounds such as resorcinol, catechol, hydroquinone, biphenol,bisphenol A, bisphenol AP (1,1-bis(4-hydroxylphenyl)-1-phenyl ethane),bisphenol F, bisphenol K and tetramethylbiphenol; diglycidyl ethers ofaliphatic glycols and polyether glycols such as the diglycidyl ethers ofC₂₋₂₄ alkylene glycols and poly(ethylene oxide) or poly(propylene oxide)glycols; polyglycidyl ethers of phenol-formaldehyde novolac resins(epoxy novolac resins), alkyl substituted phenol-formaldehyde resins,phenol-hydroxybenzaldehyde resins, cresol-hydroxybenzaldehyde resins,dicyclopentadiene-phenol resins and dicyclopentadiene-substituted phenolresins; cycloaliphatic epoxy resins, and any combination of any two ormore thereof.

Suitable epoxy resins include diglycidyl ethers of bisphenol A resinssuch as are sold by Olin Corporation under the designations D.E.R.® 330,D.E.R.® 331, D.E.R.® 332, D.E.R.® 383, D.E.R. 661 and D.E.R.® 662resins.

Commercially available diglycidyl ethers of polyglycols that are usefulinclude those sold as D.E.R.® 732 and D.E.R.® 736 by Olin Corporation.

Suitable epoxy novolac resins that are commercially available includethose sold as D.E.N.® 354, D.E.N.® 431, D.E.N.® 438 and D.E.N.® 439 fromOlin Corporation.

Suitable cycloaliphatic epoxy resins include those described in U.S.Pat. No. 3,686,359, incorporated herein by reference. Cycloaliphaticepoxy resins of particular interest are(3,4-epoxycyclohexyl-methyl)-3,4-epoxy-cyclohexane carboxylate,bis-(3,4-epoxycyclohexyl) adipate, vinylcyclohexene monoxide andmixtures thereof.

Other suitable epoxy resins include oxazolidone-containing compounds asdescribed in U.S. Pat. No. 5,112,932. In addition, an advancedepoxy-isocyanate copolymer such as those sold commercially as D.E.R. 592and D.E.R. 6508 (Olin Corporation) can be used.

The epoxy resin preferably is one or more diglycidyl ethers of abisphenol or a mixture thereof with up to 10 percent by weight ofanother epoxy resin that is not a diglycidyl ether of a bisphenol. Themost preferred epoxy resins are bisphenol-A based epoxy resins andbisphenol-F based epoxy resins. The bisphenol diglycidyl ether(s) canhave average epoxy equivalent weights of 170 to 600 or more, preferably170 to 400.

An especially preferred epoxy resin is a mixture of at least onediglycidyl ether of a polyhydric phenol, preferably bisphenol-A orbisphenol-F, having an epoxy equivalent weight of 170 to 299, especially170 to 225, and at least one second diglycidyl ether of a polyhydricphenol, again preferably bisphenol-A or bisphenol-F, this one having anepoxy equivalent weight of at least 300, preferably 310 to 600. Theproportions of the resins are preferably such that the mixture has anumber average epoxy equivalent weight of 190 to 400. The mixtureoptionally may also contain up to 20%, preferably up to 10%, of one ormore other epoxy resins.

The epoxy resin preferably will constitute at least about 25 weightpercent of the structural adhesive, more preferably at least about 30weight percent, and still more preferably at least about 40 weightpercent. The epoxy resin may constitute up to about 70 weight percent ofthe structural adhesive, more preferably up to about 60 weight percent.If any of the epoxy resins are rubber-modified, the weight of the rubbercomponent is not counted as part of the weight of the epoxy resin.

In some embodiments, the structural adhesive composition contains 30 to60, preferably 40 to 60, weight percent of a diglycidyl ether ofbisphenol A that has an epoxy equivalent weight of up to 225, and 0 to20%, preferably 2 to 15 weight percent, of a diglycidyl ether ofbisphenol A that has an epoxy equivalent weight of 400 or greater,preferably 400 to 1500. Such a structural adhesive compositionoptionally contains 0.5 to 10 weight percent of a different epoxy resinsuch as an epoxy novolac resin or an epoxy cresol novolac resin.

The toughener contains urethane and/or urea groups and has terminalcapped isocyanate groups. Such tougheners are well-known and can bemade, for example, according to any of the methods described in U.S.Pat. Nos. 5,202,390, 5,278,257, WO 2005/118734, WO 2007/003650,WO2012/091842, U. S. Published Patent Application No. 2005/0070634, U.S. Published Patent Application No. 2005/0209401, U. S. Published PatentApplication 2006/0276601, EP-A-0 308 664, EP 1 498 441A, EP-A 1 728 825,EP-A 1 896 517, EP-A 1 916 269, EP-A 1 916 270, EP-A 1 916 272 andEP-A-1 916 285.

A particular process for forming the toughener includes the steps offorming an isocyanate-terminated prepolymer, optionally chain-extendingthe prepolymer and then capping the isocyanate groups of the prepolymeror chain-extended prepolymer.

The prepolymer is formed by reacting an excess of a polyisocyanate witha 300 to 3000 equivalent weight polyol to form an isocyanate-terminatedprepolymer. The 300 to 3000 equivalent weight polyol may be, forexample, a polyether polyol, a hydroxyl-terminated butadiene homopolymeror copolymer, a hydroxyl-terminated polysiloxane, or otherhydroxyl-terminated material that preferably has a glass transitiontemperature of 0° C. or lower, preferably −20° C. or lower. If apolyether polyol, it may be a polymer of one or more of tetrahydrofuran(tetramethylene oxide), 1,2-butylene oxide, 2,3-butylene oxide,1,2-propylene oxide and ethylene oxide, with polymers or copolymers ofat least 50% of tetrahydrofuran, 1,2-butylene oxide, 2,3-butylene oxideand 1,2-propylene oxide being preferred. The polyol preferably has 2 to3, more preferably 2, hydroxyl groups per molecule.

A branching agent may be present during the formation of the prepolymer.The branching agent, for purposes of this invention, is a polyol orpolyamine compounds having a molecular weight of up to 599, preferably50 to 500, and at least three hydroxyl, primary amino and/or secondaryamino groups per molecule. If used at all, branching agents generallyconstitute no more than 10%, preferably no more than 5% and still morepreferably no more than 2% of the combined weight of the branching agentand 300 to 3000 equivalent weight polyol. Examples of branching agentsinclude polyols such as trimethylolpropane, glycerin, trimethylolethane,ethylene glycol, diethylene glycol, propylene glycol, dipropyleneglycol, sucrose, sorbitol, pentaerythritol, triethanolamine,diethanolamine and the like, as well as alkoxylates thereof having amolecular weight of up to 599, especially up to 500.

The polyisocyanate reacted with the 300 to 3000 equivalent weight polyol(and optional branching agent) may be an aromatic polyisocyanate, but itis preferably an aliphatic polyisocyanate such as isophoronediisocyanate, 1,6-hexamethylene diisocyanate, hydrogenated toluenediisocyanate, hydrogenated methylene diphenylisocyanate (H₁₂MDI), andthe like.

An excess of the polyisocyanate compound is used, so that essentiallyall of the isocyanate reactive groups of the 300 to 3000 equivalentweight polyol and branching agent (if any) are consumed and theresulting prepolymer is terminated with isocyanate groups. It isgenerally preferred to combine at least 1.5 equivalents of thepolyisocyanate per equivalent of the isocyanate-reactive materials(i.e., the 300 to 3000 molecular weight polyol and the branching agent,if any), as such a ratio minimizes the formation of materials that areadvanced in molecular weight. More preferably, from 1.5 to 2.5equivalents of the polyisocyanate are provided per equivalent of theisocyanate-reactive materials.

The prepolymer-forming reaction is performed by mixing the startingmaterials and heating them, preferably in the presence of a catalyst forthe reaction of isocyanate groups with hydroxyl groups. The reactionmixture is conveniently heated to 60 to 120° C., and the reaction isconveniently continued until a constant isocyanate content is obtained,indicating that all of the isocyanate-reactive groups in the startingmaterials have been consumed.

The resulting prepolymer preferably has an isocyanate content of 0.5 to7% by weight, more preferably 1 to 6% by weight and even more preferably1.5 to 5% by weight. In terms of number average isocyanate equivalentweight, a preferred range is 700 to 8400, a more preferred range is 840to 4200, and an even more preferred range is 1050 to 2800. Theprepolymer suitably contains, on average, at least 1.5, preferably atleast two 2.0, to about 4, preferably to about 3, and more preferably toabout 2.5 isocyanate groups per molecule.

Before capping, the prepolymer may be reacted with a chain extender toproduce a chain extended, isocyanate-terminated prepolymer. Chainextenders include polyol or polyamine compounds having a molecularweight of up to 749, preferably 50 to 500, and two hydroxyl, primaryamino and/or secondary amino groups per molecule. Examples of suitablechain extenders include aliphatic diols such as ethylene glycol,diethylene glycol, triethylene glycol, propylene glycol, dipropyleneglycol, tripropylene glycol, 1,4-butanediol, 1,6-hexane diol,cyclohexanedimethanol and the like; aliphatic or aromatic diamines suchas ethylene diamine, piperazine, aminoethylpiperazine, phenylenediamine, diethyltoluenediamine and the like, and compounds having twophenolic hydroxyl groups such resorcinol, catechol, hydroquinone,bisphenol, bisphenol A, bisphenol AP (1,1-bis(4-hydroxylphenyl)-1-phenylethane), bisphenol F, bisphenol K, bisphenol M, tetramethylbiphenol ando,o′-diallyl-bisphenol A, and the like. Among these, the compoundshaving two phenolic hydroxyl groups are preferred.

The chain extension reaction, if done, is performed in the same generalmanner as the prepolymer-forming reaction. Enough of the prepolymer ismixed with the chain extender to provide at least two equivalents ofisocyanate groups per equivalent of isocyanate-reactive groupscontributed by the chain extender. From 2 to up to 4 or more, preferablyup to 3 and more preferably up to 2.5 equivalents of isocyanate groupsmay be provided per equivalent of isocyanate-reactive groups contributedby the chain extender during the chain extension reaction.

The isocyanate groups of the chain-extended prepolymer (orchain-extended prepolymer) are then capped by reaction with a cappinggroup. Various types of capping groups are suitable including thosedescribed in U.S. Pat. Nos. 5,202,390, 5,278,257, 7,615,595, USPublished Patent Application Nos. 2005-0070634, 2005-0209401,2006-0276601 and 2010-0019539, WO 2006/128722, WO 2005/118734 and WO2005/0070634, all incorporated herein by references. Among the usefulcapping agents are:

a) Aliphatic, aromatic, cycloaliphatic, araliphatic and/orheteroaromatic monoamines that have one primary or secondary aminogroup. Examples of such capping compounds include monoalkyl amines suchas methyl amine, ethyl amine, isopropyl amine, sec-butylamine, t-butylamine; dialkyl amines such as dimethylamine, diethylamine,diisopropylamine, di-sec-butylamine, dihexylamine and dioctyl amine;cyclohexylamine or dicyclohexylamine wherein the cyclohexyl groups areoptionally substituted with one or more alkyl groups; benzylamine anddiphenylamine wherein the phenyl groups are optionally substituted withone or more alkyl groups; morpholine; N-alkylpiperadines and imidazolshaving an amine hydrogen atom.

b) phenolic compounds, including monophenols, polyphenols andaminophenols. Examples of such phenolic capping compounds includephenol, alkyl phenols that contain one or more alkyl groups that eachmay contain from 1 to 30 carbon atoms, naphthol or a halogenated phenol.Suitable polyphenols contain two or more, preferably two, phenolichydroxyl groups per molecule and include resorcinol, catechol,hydroquinone, bisphenol, bisphenol A, bisphenol AP(1,1-bis(4-hydroxylphenyl)-1-phenyl ethane), bisphenol F, bisphenol K,bisphenol M, tetramethylbiphenol and o,o′-diallyl-bisphenol A, as wellas halogenated derivatives thereof. Suitable aminophenols are compoundsthat contain at least one primary or secondary amino group and at leastone phenolic hydroxyl group. The amino group is preferably bound to acarbon atom of an aromatic ring. Examples of suitable aminophenolsinclude 2-aminophenol, 4-aminophenol, various aminonaphthols, and thelike.

c) Benzyl alcohol, which may be substituted with one or more alkylgroups on the aromatic ring;

d) Hydroxy-functional acrylate or methacrylate compounds such as2-hydroxyethylacrylate, 2-hydroxypropylacrylate, 4-hydroxybutylacrylate,2-hydroxybutylacrylate, 2-aminopropylacrylate,2-hydroxyethylmethacrylate, 2-hydroxypropylmethacrylate,4-hydroxybutylmethacrylate and 2-hydroxybutylmethacrylate;

e) thiol compounds such as alkylthiols having 2 to 30, preferably 6 to16, carbon atoms in the alkyl group, including dodecanethiol;

f) alkyl amide compounds having at least one amine hydrogen such asacetamide and N-alkylacetamide; and

g) a ketoxime.

The phenol, polyphenol and aminophenol capping agents are generallypreferable. In some embodiments, at least 90%, preferably at least 95%,more preferably at least 98%, up to 100% of the isocyanate groups of theprepolymer are capped with capping agents of one or more of these types.In such embodiments any remaining uncapped isocyanate groups may becapped with another type of capping agent.

The capping reaction can be performed under the general conditionsdescribed already with respect to the prepolymer-forming andchain-extension reactions, i.e., by combining the materials in thestated ratios and heating to 60 to 120° C., optionally in the presenceof a catalyst for the reaction of isocyanate groups with theisocyanate-reactive groups of the capping agent. The reaction iscontinued until the isocyanate content is reduced to a constant value,which is preferably less than 0.1% by weight. Preferably, fewer than 5%,preferably fewer than 1% of the isocyanate groups may remain uncapped.

The resulting toughener suitably has a number average molecular weightfrom at least 3000, preferably at least 4,000, to about 35,000,preferably to about 20,000 and more preferably to about 15,000, measuredby GPC, taking into account only those peaks that represent molecularweights of 1000 or more.

The polydispersity (ratio of weight average molecular weight to numberaverage molecular weight) of the toughener is suitably from about 1 toabout 4, preferably from about 1.5 to 2.5. The toughener suitablycontains, on average, from about 1.5, preferably from about 2.0, toabout 6, preferably to about 4, more preferably to about 3 and stillmore preferably to about 2.5, capped isocyanate groups per molecule. Anespecially preferred prepolymer contains an average of from 1.9 to 2.2capped isocyanate groups per molecule.

The toughener by itself may have a glass transition temperature of nogreater than 0° C., preferably no greater than −20° C. and morepreferably no greater than −35° C., as measured by differential scanningcalorimetry.

The toughener constitutes at least 5 weight percent, preferably at least8 weight percent or at least 10 weight percent, of the structuraladhesive composition. The toughener may constitute up to 45 weightpercent thereof, preferably up to 30 weight percent and more preferablyup to 25 weight percent. The amount of toughener that is needed toprovide good properties, particularly good low temperature properties,in any particular structural adhesive composition may depend somewhat onthe other components of the composition, and may depend somewhat on themolecular weight of the toughener.

The structural adhesive also contains a curing agent. The curing agentis selected together with any catalysts such that the adhesive exhibitsa curing temperature of at least 60° C.

Typically the structural adhesive will exhibit a characteristic curingtemperature at or above which it reacts and cures rapidly, due to thethermal activation of the curing agent, the catalyst, or both. Thecuring temperature preferably is at least 80° C., and may be at least100° C. or at least 140° C. It may be as high as, for example, 180° C.The “curing temperature” refers to the lowest temperature at which thestructural adhesive achieves at least 30% of its lap shear strength (DINISO 1465) at full cure within 2 hours. The lap shear strength at “fullcure” is measured on a sample that has been cured for 30 minutes at 180°C., which conditions represent “full cure” conditions.

The curing agent is a compound that reacts with at least two epoxygroups to form a linkage between them. Suitable curing agents includematerials such as boron trichloride/amine and boron trifluoride/aminecomplexes, dicyandiamide, melamine, diallylmelamine, guanamines such asdicyandiamide, methyl guanidine, dimethyl guanidine, trimethylguanidine, tetramethyl guanidine, methylisobiguanidine,dimethylisobiguanidine, tetramethylisbiguandidine,heptamethylisobiguanidine, hexamethylisobiguanidine, acetoguanamine andbenzoguanamine, aminotriazoles such as 3-amino-1,2,4-triazole,hydrazides such as adipic dihydrazide, stearic dihydrazide, isophthalicdihydrazide, semicarbazide, cyanoacetamide, and aromatic polyamines suchas diaminodiphenylsulphones. The use of dicyandiamide, isophthalic aciddihydrazide, adipic acid dihydrazide and/or 4,4′-diaminodiphenylsulphoneis particularly preferred.

The curing agent is used in an amount sufficient to cure thecomposition. Typically, enough of the curing agent is provided toconsume at least 80% of the epoxide groups present in the composition. Alarge excess over that amount needed to consume all of the epoxidegroups is generally not needed. Preferably, the curing agent constitutesat least about 1.5 weight percent of the structural adhesive, morepreferably at least about 2.5 weight percent and even more preferably atleast 3.0 weight percent. The curing agent preferably constitutes up toabout 15 weight percent of the structural adhesive composition, morepreferably up to about 10 weight percent, and most preferably up toabout 8 weight percent.

The structural adhesive contains a catalyst to promote the cure of theadhesive, i.e., the reaction of epoxy groups with epoxide-reactivegroups on the curing agent and other components of the adhesive. Asmentioned above, the catalyst is selected together with the curing agentto provide the heat-activated cure. The catalyst is preferablyencapsulated or otherwise a latent type which becomes active only uponexposure to elevated temperatures. Among preferred epoxy catalysts areureas such as p-chlorophenyl-N, N-dimethylurea (Monuron), 3-phenyl-1,1-dimethylurea (Phenuron), 3,4-dichlorophenyl-N, N-dimethylurea(Diuron), N-(3-chloro-4-methylphenyl)-N′,N′-dimethylurea (Chlortoluron),tert-acryl- or alkylene amines like benzyldimethylamine,2,4,6-tris(dimethylaminomethyl)phenol, piperidine or derivativesthereof, various aliphatic urea compounds such as are described in EP 1916 272; C₁-C₁₂ alkylene imidazole or N-arylimidazoles, such as2-ethyl-2-methylimidazol, or N-butylimidazol and 6-caprolactam. Apreferred catalyst is 2,4,6-tris(dimethylaminomethyl)phenol integratedinto a poly(p-vinylphenol) matrix (as described in European patent EP 0197 892), or 2,4,6-tris(dimethylaminomethyl)phenol integrated into anovolac resin, including those described in U.S. Pat. No. 4,701,378.

The catalyst may be present in an amount of at least about 0.1 weightpercent of the structural adhesive, more preferably at least about 0.5weight percent. Preferably, the catalyst constitutes up to about 4weight percent of the structural adhesive, more preferably up to about1.5 weight percent, and most preferably up to about 0.9 weight percent.

The structural adhesive contains at least one mineral filler in the formof fibers having a diameter of 1 to 50 μm (D50, as measured bymicroscopy) and an aspect ratio of at least 6. The diameter of thefibers may be 3 to 25 μm or 5 to 20 μm, and the aspect ratio may be atleast 8 or at least 9, at least 12, at least 15 or at least 20. Theaspect ratio of the fibers may be, for example, up to 100, up to 50, upto 40, up to 30 or up to 20.

The diameter of the fiber is taken as that of a circle having the samecross-sectional area as the fiber. The aspect ratio is the fiber lengthdivided by the diameter.

The fibers may have a distribution of aspect ratios in the range of 6 orgreater, such as from 6 to 50. When such a distribution of fibers ispresent, in some embodiments at least 10% by weight, at least 25% byweight or at least 50% by weight have an aspect ratio of at least 20,preferably 20 to 50 or 20 to 40, with the remainder of the fibers havingan aspect ratio of at least 6 but less than 20.

The fibers may be, for example, calcium carbonate, calcium oxide, talc,carbon black, glass, an aluminosilicate, a calcium silicate, mica,hydrated aluminum oxide, or a naturally occurring clay such asbentonite, wollastonite or kaolin.

The fibrous mineral filler may constitute 1 to 40% of the total weightof the structural adhesive composition. In some embodiments, itconstitutes at least 5% or at least 7.5% of the weight of the structuraladhesive composition, and may constitute up to 30%, up to 25%, up to 20%or up to 15% of the weight thereof. In addition to having a beneficialeffect on the elastic modulus of the cured adhesive, this filler alsomay perform one or more other functions, such as (1) modifying therheology of the structural adhesive in a desirable way, (2) reducingoverall cost per unit weight, (3) absorbing moisture or oils from thestructural adhesive or from a substrate to which it is applied, and/or(4) promoting cohesive, rather than adhesive, failure. The presence ofthe fibers in the cured adhesives also has been found to lead to anincrease in dynamic impact peel strength (as measured by the wedgeimpact method of ISO 11343), compared to the case of an otherwise likeadhesive which instead contains a low aspect ratio filler (L/D<6) at thesame weight content. This last benefit is more pronounced when at leastof the portion of the fibers has an aspect ratio of at least 20.

The structural adhesive of the invention may contain various other,optional ingredients, in addition to those described above.

At least one low (less than 6, preferably less 3, more preferably lessthan 2 length/diameter ratio) aspect ratio filler may be present in thestructural adhesive, in addition to the fibrous mineral described above.This low molecular weight filler may be made of any of the materialsdescribed above with regard to the fibrous mineral filler, as well asvarious types of polymeric fibers, expandable microballoons andnon-expandable microballoons. The low aspect ratio filler(s) mayconstitute 0.1 to 30 weight percent of the structural adhesivecomposition. Preferably, the low aspect ratio fillers constitute 15% orless of the weight of the structural adhesive composition. Morepreferably, the low aspect ratio filler(s) and fibrous mineral fillertogether constitute 10 to 40, more preferably 10 to 35 and still morepreferably 10 to 30 percent of the weight of the structural adhesivecomposition.

In addition to the fibrous mineral filler and the low aspect ratiofiller, the structural adhesive composition may contain up to 10% byweight, preferably 1 to 6% by weight, of one or more desiccants such asfumed silica, hydrophobically modified fumed silica, silica gel,aerogel, various zeolites and molecular sieves, and the like.

The structural adhesive composition may include a rubber component,separate from the toughener described above. The rubber component doesnot include capped isocyanate groups. The rubber component may be, forexample, a liquid rubber, preferably having two or more epoxide-reactivegroups such as amino or preferably carboxyl groups. It is preferred thatat least a portion of the liquid rubber has a glass transitiontemperature (T_(g)) of −40° C. or lower, especially −50° C. or lower, asmeasured by differential scanning calorimetry. Such a liquid rubbercomponent may be entirely or partially reacted with a portion of theepoxy resin component.

Such a liquid rubber may be a homopolymer or copolymer of a conjugateddiene, especially a diene/nitrile copolymer. The conjugated diene rubberis preferably butadiene or isoprene, with butadiene being especiallypreferred. The preferred nitrile monomer is acrylonitrile. Preferredcopolymers are butadiene-acrylonitrile copolymers. The rubberspreferably contain, in the aggregate, no more than 30 weight percentpolymerized unsaturated nitrile monomer, and preferably no more thanabout 26 weight percent polymerized nitrile monomer. The liquid rubberpreferably contains from about 1.5, more preferably from about 1.8, toabout 2.5, more preferably to about 2.2, of epoxide-reactive terminalgroups per molecule, on average. Carboxyl-terminated rubbers arepreferred. The molecular weight (Me) of the rubber is suitably fromabout 2000 to about 6000, more preferably from about 3000 to about 5000.Suitable carboxyl-functional butadiene and butadiene/acrylonitrilerubbers are commercially available from Noveon under the tradenamesHycar® 2000X162 carboxyl-terminated butadiene homopolymer, Hycar®1300X31, Hycar®1300X8, Hycar®1300X13, Hycar®1300X9 and Hycar® 1300X18carboxyl-terminated butadiene/acrylonitrile copolymers. A suitableamine-terminated butadiene/acrylonitrile copolymer is sold under thetradename Hycar® 1300X21.

Other suitable rubber materials include amine-terminated polyethers,fatty acids (which may be dimerized or oligomerized), and elastomericpolyester.

Another type of rubber that may be present in the structural adhesivecomposition is a core-shell rubber. The core-shell rubber is aparticulate material having a rubbery core. The rubbery core preferablyhas a T_(g) of less than −20° C., more preferably less than −50° C. andeven more preferably less than −70° C. by differential scanningcalorimetry. The T_(g) of the rubbery core may be well below −100° C.The core-shell rubber also has at least one shell portion thatpreferably has a T_(g) of at least 50° C. by differential scanningcalorimetry. The core of the core-shell rubber may be a polymer orcopolymer of a conjugated diene such as butadiene, or a lower alkylacrylate such as n-butyl-, ethyl-, isobutyl- or 2-ethylhexylacrylate, ormay be a silicone rubber. The shell polymer, which is optionallychemically grafted or crosslinked to the rubber core, is preferablypolymerized from at least one lower alkyl methacrylate such as methyl-,ethyl- or t-butyl methacrylate. Homopolymers of such methacrylatemonomers can be used. Further, up to 40% by weight of the shell polymercan be formed from other monovinylidene monomers such as styrene, vinylacetate, vinyl chloride, methyl acrylate, ethyl acrylate, butylacrylate, and the like. The molecular weight of the grafted shellpolymer is generally between 20,000 and 500,000. Examples of usefulcore-shell rubbers include those described in EP 1 632 533 A1 and thosesold by Kaneka Corporation under the designation Kaneka Kane Ace,including Kaneka Kane Ace MX 156 and Kaneka Kane Ace MX 120 core-shellrubber dispersions.

The total rubber content of the structural adhesive of the invention canrange from as little as 0 weight percent to as high as 30 weightpercent, based on the total weight of the adhesive. A preferred rubbercontent is up to 20 weight percent, up to 15 weight percent or up to 5weight percent. No portion of the toughener is considered in calculatingtotal rubber content.

A monomeric or oligomeric, addition polymerizable, ethylenicallyunsaturated material is optionally present in the structural adhesivecomposition. This material should have a molecular weight of less thanabout 1500. This material may be, for example, an acrylate ormethacrylate compound, an unsaturated polyester, a vinyl ester resin, oran epoxy adduct of an unsaturated polyester resin. A free radicalinitiator can be included in the structural adhesive composition aswell, in order to provide a source of free radicals to polymerize thismaterial. The inclusion of an ethylenically unsaturated material of thistype provides the possibility of effecting a partial cure of thestructural adhesive through selective polymerization of the ethylenicunsaturation.

The structural adhesive composition can further contain other additivessuch as dimerized fatty acids, diluents, plasticizers, extenders,pigments and dyes, fire-retarding agents, thixotropic agents, expandingagents, flow control agents, adhesion promoters and antioxidants.Suitable expanding agents include both physical and chemical typeagents. The structural adhesive may also contain a thermoplastic powdersuch as polyvinylbutyral or a polyester polyol, as described in WO2005/118734.

The foregoing structural adhesive composition is formed into a layer ata bondline between two substrates to form an assembly, and thestructural adhesive layer is cured at the bondline to form an adhesivebond between the two substrates.

The structural adhesive composition can be applied to the substrates byany convenient technique. It can be applied cold or be applied warm ifdesired. It can be applied manually and/or robotically, using forexample, a caulking gun, other extrusion apparatus, or jet sprayingmethods. Once the structural adhesive composition is applied, thesubstrates are contacted such that the adhesive is located at a bondlinebetween the substrates.

After application, the structural adhesive is cured by heating it to ator above its curing temperature. Generally, this temperature is at least60° C., and is preferably 80° C. or above, more preferably about 140° C.or above. Preferably, the temperature is about 220° C. or less, and morepreferably about 180° C. or less.

The structural adhesive of the invention can be used to bond a varietyof substrates together including wood, metal, coated metal, aluminum, avariety of plastic and filled plastic substrates, fiberglass and thelike. In one preferred embodiment, the structural adhesive is used tobond parts of automobiles or other vehicles. Such parts can be steel,coated steel, galvanized steel, aluminum, coated aluminum, plastic andfilled plastic substrates.

An application of particular interest is in bonding vehicle framecomponents to each other or to other components. The frame componentsare often metals such as cold rolled steel, galvanized metals, oraluminum. The components to be bonded to the frame components can alsobe metals as just described, or can be other metals, plastics, compositematerials, and the like.

Assembled automotive frame members are usually coated with a coatingmaterial that requires a bake cure. The coating is typically baked attemperatures that may range from 140° C. to over 200° C. In such cases,it is often convenient to apply the structural structural adhesive tothe frame components, then apply the coating, and cure the structuraladhesive at the same time the coating is baked and cured.

The cured adhesive in some embodiments exhibits an elastic modulus of atleast 2200 MPa, preferably at least 2500 MPa and more preferably atleast 3000 MPa, when cured for 30 minutes at 180° C. and tested inaccordance with DIN EN ISO 527-1. The elastic modulus may be as much as5000 MPa or more. The cured structural adhesive in some embodiments alsoexhibits a dynamic impact peel strength of at least 20 N/mm, preferablyat least 25 N/mm, to as much as 50 N/mm or more, when applied betweenoily 1.0 mm thick HC420LAD+Z100 steel coupons, cured at 180° C. for 30minutes and then tested at 23° C. in accordance with the ISO 11343 wedgeimpact method at 23° C., as described in the following examples.

The following examples are provided to illustrate the invention but arenot intended to limit the scope thereof. All parts and percentages areby weight unless otherwise indicated.

In the following examples:

Epoxy Resin A is a liquid diglycidyl ether of bisphenol A having anepoxy equivalent weight of about 187.

Epoxy Resin B is a mixture of solid and liquid diglycidyl ethers ofbisphenol A. The mixture has an epoxy equivalent weight of about 240.

Epoxy Resin C is an epoxy novolac resin having an epoxy equivalentweight of about 179.

Epoxy Resin D is an epoxy-functional diluent.

Toughener A contains blocked isocyanate groups. It is prepared by mixing54.79 parts of a 2000 molecular weight polytetrahydrofuran and 13.65parts of a 2800 molecular weight, hydroxyl-terminated polybutadienerubber at 120° C., cooling the mixture to 60° C., adding 15.09 parts ofisophorone diisocyanate and a tin urethane catalyst and heating theresulting reaction mixture to 85° C. for 45 minutes under nitrogen.Then, 5.74 parts of o,o-diallylbisphenol A are added, and the mixture isstirred for 120 minutes under vacuum in a 100° C. bath. 10.65 parts ofcardanol are added and the mixture is stirred for 240 minutes undervacuum in a 105° C. bath.

Toughener B contains blocked isocyanate groups. It is prepared by mixing57.58 parts of a 2000 molecular weight polytetrahydrofuran and 14.39parts of a 2800 molecular weight hydroxyl-terminated polybutadienerubber at 120° C., cooling the mixture to 60° C., adding 11.54 parts ofhexamethylene diisocyanate and a tin urethane catalyst and heating theresulting reaction mixture to 85° C. for 45 minutes under nitrogen.Then, 5.74 parts of o,o′-diallylbisphenol A are added, and the mixtureis stirred for 120 minutes under vacuum in a 100° C. bath. 10.58 partsof cardanol are added and the mixture is stirred for 240 minutes undervacuum in a 105° C. bath.

GLYEO is a commercial grade of 3-glycidyloxypropyltriethoxysilane.

EP796 is tris (2,4,6-dimethylaminomethyl)phenol in a poly(vinylphenol)matrix.

The Calcium Carbonate is a low aspect ratio product availablecommercially as Omya BSH, from Omya GmbH.

The Calcium Oxide is a low aspect ratio product from Lhoist Group.

Microspheres A are hollow glass spheres, 90% of which having a particlesize of 10 to 105 μm and a density of 0.25 g/cc commercially availablefrom 3M.

Microspheres B are hollow glass spheres having a particle size of 10 to40 μm and a density of 0.46 g/cc, commercially available from 3M.

Wollastonite A has an aspect ratio of about 13 and a fiber diameter ofabout 12 μm. It is sold as Nyglos 8 by NYCO Minerals.

Wollastonite B has an aspect ratio of about 9 and a fiber diameter ofabout 7 μm. It is sold as Nyglos 4W by NYCO Minerals.

Wollastonite C is a powder grade wollastonite having an aspect ratio of5 and a median particle diameter of 20 μm. It is sold as Nyad 200 byNYCO Minerals.

Wollastonite D is a powder grade wollastonite having an aspect ratio of3 and a median particle diameter of 3 μm. It is sold as Nyad 5000 byNYCO Minerals.

Glass Fiber A has an aspect ratio of 11 and a fiber diameter of about 11μm. It is sold as Lanxess MF7982 (19/346) by Lanxess.

Glass Fiber B has an aspect ratio of 15 and a fiber diameter of about 14μm. It is sold as Lanxess MF7982 by Lanxess.

Glass Fiber C has an aspect ratio of 29 and a fiber diameter of about 16μm. It is sold as Fibertec MF 6608 by Fibertec Inc.

Glass Fiber D has an aspect ratio of 18 and a fiber diameter of about 16μm. It is sold as Fibertec MF 6616 by Fibertec Inc.

Carbon Fiber A has an aspect ratio of 11 and a fiber diameter of about 7μm. It is sold as Sigrafill C C30 M 080 by SGL Carbon.

Carbon Fiber B has an aspect ratio of 21 and a fiber diameter of about 7μm. It is sold as Sigrafill C C30 M 151 by SGL Carbon.

EXAMPLE 1 AND COMPARATIVE SAMPLES A-D

Structural Adhesive Example 1 and Comparative Samples A-D are preparedby blending ingredients as indicated in Table 1:

TABLE 1 Parts By Weight Comp. Comp. Comp. Comp. Component Samp. A Samp.B Samp. C Samp. D Ex. 1 Epoxy Resin A 67.46 60.64 60.64 60.64 60.64Toughener A 19.0 19.0 19.0 19.0 19.0 GLYEO 0.6 0.6 0.6 0.6 0.6 Colorant0.4 0.4 0.4 0.4 0.4 Dicyanamide 6.74 6.06 6.06 6.06 6.06 EP796 0.8 0.80.8 0.8 0.8 Hydrophobic 5.0 5.0 5.0 5.0 5.0 Fumed Silica Calcium Oxide 07.5 0 0 0 Wollastonite A 0 0 0 0 7.5 Wollastonite C 0 0 7.5 0 0Wollastonite D 0 0 0 7.5 0

Test samples for tensile strength, elongation and elastic modulusmeasurements are made by curing a portion of each sample for 30 minutesat 180° C. Test specimens are cut from the cured samples and evaluatedaccording to DIN EN ISO 527-1.

Impact peel testing is performed for each adhesive sample. Thesubstrates are 1.0 mm-thick HC420LAD+Z100 steel coupons. The testcoupons for the impact peel testing are 90 mm×20 mm with a bonded areaof 30×20 mm. They are cleaned with heptane and then re-greased bydipcoating them into a 9:1 by volume solution of heptane and a corrosionprevention lubricant (Anticorit PL 3802-39S). The adhesive sample isthen applied to the bond area of one coupon and squeezed onto the othercoupon to prepare each test specimen, with spacers present to maintainan adhesive layer thickness of 0.2 mm. The assembled test specimens arecured at 180° C. for 30 minutes. The impact peel testing is performed inaccordance with ISO 11343 wedge impact method. Testing is performed atan operating speed of 2 m/sec with samples at a temperature of 23° C.

Lap shear specimens are made using coupons of the same steel, exceptthey are 1.2 mm thick. The specimens are made by sprinkling glass beads(0.2 mm diameter) onto one of the coupons, applying the adhesive sample,and then positioning the second coupon over top the adhesive. The bondedarea in each case is 25×10 mm, and the adhesive layer thickness iscontrolled by the glass beads to 0.2 mm. The test specimens are curedfor 30 minutes at 180° C. and evaluated for lap shear strength inaccordance with DIN ISO 1465. Testing is performed at 23° C. and a testspeed of 10 mm/minute.

Viscosity and yield stress are measured on a Bohlin CS-50 rheometer, C/P20, up/down 0.1-20 s⁻¹, with data evaluated according to the Cassonmodel.

Results of this testing are as indicated in Table 2.

TABLE 2 Comp. Comp. Comp. Comp. Samp. A Samp. B Samp. C Samp. D Ex. 1Filler None CaO Woll. C Woll. D Woll. A Filler Aspect N/A ~1 5 3 13Ratio Property Elastic 2035 2067 2205 2162 2733 Modulus, MPa Tensile47.6 44.2 46.5 46.5 43.6 Strength, MPa Elongation 9.5 7.6 7.3 8.2 6.1 atBreak, % Impact Peel 32.0 31.9 31.1 31.0 32.2 Str., N/mm Lap Shear 40.637.8 39.5 40.3 38.5 Str., MPa Lap Shear 2.2 1.9 1.9 2.1 2.0 Elongation,% Yield stress, 63.6 78.8 86.5 85.5 56.1 45° C., Pa Viscosity, 18.0 23.922.9 24.2 26.8 45° C., Pa · s CaCO₃—calcium carbonate; CaO—calciumoxide; Woll. A—Wollastonite A; Woll. C—Wollastonite C; Woll.D—Wollastonite D.

Comparative Sample A is an unfilled control. In each of ComparativeSamples B-D and in Example 1, the Comparative Sample A formulation ismodified by adding 7.5 parts of a filler and removing an equal quantityof epoxy resin and dicyanamide. The amount of the toughener is keptconstant across these samples.

Comparative Sample B shows the effect of adding a low aspect ratiocalcium oxide filler. The addition of this filler has very little effecton any of the properties except for increasing yield stress andviscosity mildly.

The addition of low aspect ratio wollastonite as in Comparative SamplesC and D results in a small (6-8.5%) increase in elastic modulus. Likethe calcium oxide, these fillers increase yield stress and viscositysomewhat, but have little other effect on the properties of the curedadhesive.

Example 1 contains a high aspect ratio wollastonite filler. The elasticmodulus is increased by 34% over Comparative Sample A, which is severaltimes the increase seen with the fillers of Comparative Samples B, C andD. Other tensile properties, lap shear strength and, surprisingly,impact peel resistance are not significantly different than those ofComparative Samples A-D. Yield stress and viscosity are slightly higherthan the filled comparative samples.

EXAMPLES 2-4 AND COMPARATIVE SAMPLES A, E, F AND G

Structural Adhesive Examples 2-4 and Comparative Samples E, F and G areprepared by blending ingredients as indicated in Table 3:

TABLE 3 Parts By Weight Component Comp. E Comp. F Comp. G Ex. 2 Ex. 3Ex. 4 Epoxy Resin A 53.4 53.82 53.82 53.82 53.82 33.17 Epoxy Resin B 0 00 0 0 16.59 Toughener A 19.0 19.0 19.0 19.0 19.0 19.0 GLYEO 0.6 0.6 0.60.6 0.6 0.6 Colorant 0.4 0.4 0.4 0.4 0.4 0.4 Dicyanamide 4.80 5.38 5.385.38 5.38 4.58 EP796 0.8 0.8 0.8 0.8 0.8 0.8 Hydrophobic Fumed 5.0 5.05.0 5.0 5.0 5.0 Silica Calcium Carbonate 7.0 0 0 0 0 0 Calcium Oxide 6.515.0 0 0 0 6.0 Microspheres A 2.5 0 0 0 0 0 Microspheres B 0 0 0 0 03.86 Wollastonite A 0 0 0 15.0 0 10 Wollastonite B 0 0 0 0 15.0 0Wollastonite D 0 0 15.0 0 0 0

These adhesives are tested in the same manner as described with respectto Example 1 and Comparative Samples A-D. Results are as indicated inTable 4. The results for Comparative Sample A are repeated forreference.

TABLE 4 Comp. A Comp. E Comp. F Comp. G Ex. 2 Ex. 3 Ex. 4 Filler Type,None Mix¹, CaO, Woll. D, Woll. A, Woll. B, Woll. Mix², Amount 16 wt. %15 wt. % 15 wt. % 15 wt. % 15 wt. % 19.86 wt. % Properties Elastic 20352121 2247 2483 3418 3470 2899 Modulus, MPa Tensile 47.6 37.9 41.4 43.644.5 51.9 44.2 Strength, MPa Elongation 9.5 3.8 5.6 6.1 2.9 4.0 3.9 atBreak, % Impact Peel 32.0 25.8 29.7 30.4 30.9 30.4 27.0 Str., N/mm LapShear 40.6 31.8 36.1 38.0 35.7 38.0 36.4 Str., MPa Lap Shear 2.2 1.5 1.81.9 1.7 1.9 1.8 Elongation, % Yield stress, 63.6 107.2 106.0 102.3 66.2124.3 89.6 45° C., Pa Viscosity, 18.0 44.8 37.5 39.3 37.8 32.3 57.8 45°C., Pa · s ¹Mixture of calcium carbonate, calcium oxide and hollow glassspheres. ²Mixture of Wollastonite A, calcium carbonate and glassmicrospheres.

Comparative Sample A is again the unfilled baseline structural adhesive.In each of Comparative Samples E, F and G and Examples 2-4, theComparative Sample A formulation is modified by adding filler andremoving an equal quantity of epoxy resin and dicyanamide. The amount ofthe toughener is kept constant across these samples. The epoxy resincomponent also is changed in Example 4.

Comparative Samples E, F and G shows the effect of adding low aspectratio fillers. Comparative Example G, which contains 15 weight-% a lowaspect ratio wollastonite filler, exhibits an increase in elasticmodulus of about 22%, compared to the baseline case. This increase inelastic modulus is far less than seen with Example 1, which containsonly 7.5 weight-% of the high aspect ratio wollastonite filler.Comparative Examples E and F show only a small benefit in elasticmodulus. All of Comparative Samples E, F and G show some loss in impactpeel resistance and lap shear strength.

Examples 2 and 3 each exhibit increases in elastic modulus of about 70%,compared to the baseline case. Other properties are close to or betterthan Comparative Samples E, F and G. Examples 2 and 3, in comparisonwith Comparative Samples E, F and G, clearly demonstrate the large andunexpected effect of selecting a high aspect ratio filler. Examples 2and 3 in comparison with Comparative Sample G shows that this effect isnot due to the differences in the type of filler.

Example 4 is formulated somewhat differently than the other adhesives,but nonetheless exhibits a large improvement in elastic modulus comparedto any of the Comparative Samples.

EXAMPLE 5 AND COMPARATIVE SAMPLE H

Structural Adhesive Example 5 and Comparative Sample H are prepared byblending ingredients as indicated in Table 5:

TABLE 5 Parts By Weight Component Comp. Sample H Ex. 5 Epoxy Resin A14.0 Epoxy Resin B 30.0 Epoxy Resin C 3.0 Epoxy Resin D 2.0 Toughener B19.5 GLYEO 0.55 Colorant 0.35 Dicyanamide 4.70 Curing Accelerants 1.5Polyvinyl butyral terpolymer 0.65 Hydrophobic Fumed Silica 5.70 CalciumCarbonate 7.6 0 Calcium Oxide 6.5 6.5 Microspheres A 1.75 1.75 0.2 mmglass beads 2.0 2.0 Wollastonite A 0 7.6

These adhesives are tested in the same manner as described above, withresults as indicated in Table 6. The substrates for the dynamic impactpeel strength testing are 0.75 mm grade DX56 D+Z100 MB hot dipped zinccoated steel and 0.75 mm DC04 ZE50/50 electrolytically tine coatedsteel, each from Thyssen Krupp.

TABLE 6 Comp. H Ex. 5 Filler Type, Amount Low aspect ratio mixture. Highaspect ratio wollastonite, low aspect ratio calcium oxide, microbeadsProperties Elastic Modulus, 1837 2508 MPa Tensile Strength, 31.0 35.0MPa Elongation at 4.6 3.7 Break, % Impact Peel Str., 31.8 30.0 N/mmYield stress, 249 408 45° C., Pa Viscosity, 65.0 70 45° C., Pa · s¹Mixture of calcium carbonate, calcium oxide and hollow glass spheres.²Mixture of Wollastonite A, calcium carbonate and glass microspheres.

Once again, the inclusion of a high aspect ratio mineral filler leads toa large improvement in elastic modulus while having only a small effecton other properties. In Example 5, the benefits of the high aspectfiller are seen even when additional, low aspect ratio fillers arepresent.

EXAMPLES 6-15

Example 1 is repeated 10 times, in each instance replacing thewollastonite fibers with glass or carbon fibers as indicated in Table 7.Results of testing of the resulting cured adhesives are as indicated inTable 8.

TABLE 7 Parts by Weight Component Ex. 6 Ex. 7 Ex. 8 Ex. 9 Ex. 10 Ex. 11Ex. 12 Ex. 13 Ex. 14 Ex. 15 Epoxy Resin A 53.82 53.82 47.00 40.18 53.8260.64 53.82 47.00 58.37 53.82 Toughener A 19.00 19.00 19.00 19.00 19.0019.00 19.00 19.00 19.00 19.00 GLYEO 0.60 0.60 0.60 0.60 0.60 0.60 0.600.60 0.60 0.60 Colorant 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.400.40 Dicyanamide 5.38 5.38 4.70 4.02 5.38 6.06 5.38 4.70 5.83 5.38 EP7960.80 0.80 0.80 0.80 0.80 0.80 0.80 0.80 0.80 0.80 Hydrophobic 5.00 5.005.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00 Fumed Silica Glass Fiber A 15 00 0 0 0 0 0 0 0 Glass Fiber B 0 15 22.50 30.00 0 0 0 0 0 0 Glass Fiber C0 0 0 0 15 0 0 0 0 0 Carbon Fiber A 0 0 0 0 0 7.5 15 22.5 0 0 CarbonFiber B 0 0 0 0 0 0 0 0 10 15

TABLE 8 Designation Ex. 6 Ex. 7 Ex. 8 Ex. 9 Ex. 10 Ex. 11 Ex. 12 Ex. 13Ex. 14 Ex. 15 Fiber loading, % 15   15   22.5 30   15    7.5 15   22.510   15   Fiber Aspect Ratio, 11   15   15   15   29   11   11   11  21   21   diameter (μm) (14)   (14)   (14)   (14)   (16)   (7)  (7) (7)  (7)  (7)  Properties Elastic 3083    3191    3217    3787   2786    3900    4775    5663    3779    4056    Modulus, MPa Tensile49.7 52.6 50.4 50.1 51.6 60.8 68.3 63.8 58.9 58.0 Strength, MPaElongation at  4.3  4.5  4.05  2.8  5.15  4.2  3.7  2.8  4.8  5.0 Break,% Impact Peel 32.6 36.0 31.2 29.3 35.8 34.2 32.9 23.2 33.6 32.5 Str.,N/mm Lap Shear 35.0 37.7 32.2 27.7 41.4 39.3 36.0 28.8 40.1 37.1 Str.,MPa Lap Shear 35.0  1.8  1.44  1.7  1.8  2.1  1.9  1.2  1.8  2.0Elongation, % Yield stress, 122.4  108.4  153.6  127   145.1  71.9 75.6ND 78.0 83.5 45° C., Pa Viscosity, 33.9 32.1 46.6 155   61.5 48.2 38.7ND 28.9 37.1 45° C., Pa · s

As with previous examples of the invention, Examples 6-15 demonstratehigh elastic moduli, compared with Comparative Samples A, E, F and G,even at high fiber loadings. Impact peel strengths are generallycomparable to or better than those of the Comparative Samples, atequivalent fiber loadings.

What is claimed is:
 1. A one-component epoxy structural adhesivecomprising in admixture: A) at least one epoxy resin; B) a reactivetoughener containing urethane and/or urea groups and capped isocyanategroups; C) one or more epoxy curing agents; D) 1 to 40 weight percent,based on the total weight of the epoxy structural adhesive, of a mineralfiller in the form of fibers having a diameter of 3 μm to 25 μm and anaspect ratio of at least 6, wherein the mineral filler in the form offibers is wollastonite, and the diameter is D50 as measured bymicroscopy, and E) one or more epoxy curing catalysts, wherein thecuring agent(s) and epoxy curing catalyst(s) are selected together suchthat the structural adhesive exhibits a curing temperature of at least60° C., wherein the curing temperature is the lowest temperature atwhich the structural adhesive achieves at least 30% of its lap shearstrength according to DIN ISO 1465 at full cure within 2 hours.
 2. Theone-component epoxy structural adhesive of claim 1, which contains 5 to20 weight percent of component D).
 3. The one-component epoxy structuraladhesive of claim 2, wherein the aspect ratio of the mineral filler inthe form of fibers has an aspect ratio of at least
 9. 4. Theone-component epoxy structural adhesive of claim 2, which contains 40 to70% by weight of component A), and component A) includes one or morebisphenol diglycidyl ethers having an epoxy equivalent weight of 170 to400.
 5. The one-component epoxy structural adhesive of claim 2, whichcontains 10 to 30 weight percent of the toughener.
 6. The one-componentepoxy structural adhesive of claim 2, wherein the isocyanate groups ofthe toughener are capped with a phenol, polyphenol or aminophenol. 7.The one-component epoxy structural adhesive of claim 2 wherein thetoughener is made in a process that includes the steps of forming anisocyanate-terminated prepolymer by reacting an excess of apolyisocyanate with a 300 to 3000 equivalent weight polyol to form aprepolymer, optionally chain-extending the prepolymer and then cappingthe isocyanate groups of the prepolymer or chain-extended prepolymer. 8.The one-component epoxy structural adhesive of claim 2 wherein thecuring agent includes one or more dicyandiamide, methyl guanidine,dimethyl guanidine, trimethyl guanidine, tetramethyl guanidine,methylisobiguanidine, dimethylisobiguanidine, tetramethylisbiguandidine,heptamethylisobiguanidine, hexamethylisobiguanidine, acetoguanamine andbenzoguanamine.
 9. The one-component epoxy structural adhesive of claim2, wherein the catalyst includes one or more ofp-chlorophenyl-N,N-dimethylurea, 3-phenyl-1,1-dimethylurea,3,4-dichlorophenyl-N,N-dimethylurea,N-(3-chloro-4-methylphenyl)-N′,N′-dimethylurea, benzyldimethylamine,2,4,6-tris(dimethylaminomethyl)phenol, piperidine,2-ethyl-2-methylimidazol, N-butylimidazol, 6-caprolactam and2,4,6-tris(dimethylaminomethyl)phenol.
 10. The one-component epoxystructural adhesive of claim 2 which further contains at least one lowaspect ratio filler, wherein the aspect ratio of the low aspect ratiofiller is 6 or less.
 11. The one-component epoxy structural adhesive ofclaim 2 which is devoid of a rubber that does not contain cappedisocyanate groups.
 12. The one-component epoxy structural adhesive ofclaim 2, which exhibits an elastic modulus of at least 2200 MPa whencured for 30 minutes at 180° C. and tested in accordance with DIN EN ISO527-1.
 13. The one-component epoxy structural adhesive of claim 2, whichexhibits an elastic modulus of at least 2500 MPa when cured for 30minutes at 180° C. and tested in accordance with DIN EN ISO 527-1. 14.The one-component epoxy structural adhesive of claim 2 which exhibits adynamic impact peel strength of at least 20 N/mm when applied betweenoily 1.0 mm thick HC420LAD+Z100 steel coupons, cured at 180° C. for 30minutes and then tested at 23 C in accordance with the ISO 11343 wedgeimpact method at 23° C.
 15. The one-component epoxy structural adhesiveof claim 2 which exhibits a dynamic impact peel strength of at least 25N/mm when applied between oily 1.0 mm thick HC420LAD+Z100 steel coupons,cured at 180° C. for 30 minutes and then tested at 23 C in accordancewith the ISO 11343 wedge impact method at 23° C.
 16. The one-componentepoxy structural adhesive of claim 2, wherein at least 10 weight-% ofcomponent D) has an aspect ratio of at least
 20. 17. The one-componentepoxy structural adhesive of claim 19 which exhibits a dynamic impactpeel strength of at least 30 N/mm when applied between oily 1.0 mm thickHC420LAD+Z100 steel coupons, cured at 180° C. for 30 minutes and thentested at 23 C in accordance with the ISO 11343 wedge impact method at23° C.